Monitors are used so that performers can hear themselves while performing to be able to know how well they are working the microphones and to be able to adjust their pitch and intonation during a performance. Mono earphone monitors tend not to be preferred by performers as the resulting sound is "too close" and inside the performer's head. Monitor mixes are difficult to do in stereo because of the many problems encountered by the sound mixing engineer in trying to create live stereo monitor mixes while maintaining an evenly balanced stereo image. This task becomes more complicated as the number of performers and requested special effects for individual performers grows.
If a signal comes out of only one channel, it is placed squarely on that channel, hard right or hard left. If the signal comes out of both of channels equally, it sounds as if it is in the center. If a signal comes out of the channels equally but with the driven elements out of polarity with respect to each other, it sounds as though it is coming from a wide space behind the performer. This is the psycho acoustic pseudo stereo effect.
This psycho acoustic pseudo stereo effect is known in the art and has been described in many patents including U.S. Pat. Nos. 4,068,093; U.S. Pat. No. Re. 25,652 and U.S. Pat. No. 3,924,072.